Note 6 - Financial Instruments and Fair Value (Details Textual)	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2011 USD ($)
Asset Impairment Charges		$ 469,000					$ 8,247,000			$ 427,000	$ 1,000,000
Impairment of Real Estate				$ 400,000	$ 7,100,000
Number of Real Estate Properties Sold During Period		2	1					4
Real Estate Held-for-sale		$ 1,200,000	$ 1,000,000					$ 6,100,000
Number of Real Estate Properties Put Back into Service	1
Real Estate Previously Held-for-sale Put Back Into Service	$ 1,400,000
Auction Rate Securities [Member]
Available-for-sale Securities	6,375,000					6,375,000		6,231,000
Available-for-sale Securities, Amortized Cost Basis	$ 6,375,000					$ 6,375,000		6,700,000
Auction Rate Securities Redeemed		$ 325,000						$ 450,000	$ 275,000	$ 150,000
Derivative, Basis Spread on Variable Rate	2.25%					2.25%